MAX1845 Maxim, MAX1845 Datasheet - Page 18

• Manufacturer Part Number: MAX1845
• Description: Dual / High-Efficiency / Step-Down Controller with Accurate Current Limit
• Manufacturer: Maxim

Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (I
The inductor ripple current also impacts transient-
response performance, especially at low V
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The amount of output sag is also a function of the maxi-
mum duty factor, which can be calculated from the on-
time and minimum off-time:
where:
where minimum off-time = 400ns typ (Table 4).
The amount of overshoot during a full-load to no-load
transient due to stored inductor energy can be calculat-
ed as:
where I
For most applications, set the MAX1845 current limit by
the following procedure:
1) Determine the minimum (valley) inductor current
2) The sense resistor determines the achievable cur-
Extremely cost-sensitive applications that do not
require high-accuracy current sensing can use the on-
resistance of the low-side MOSFET switch in place of
the sense resistor by connecting CS_ to LX_ (Figure
18
DUTY
R
(IL
large, and load current is maximum. The minimum
inductor current is I
rent (Figure 4).
rent-limit accuracy. There is a trade-off between cur-
rent-limit accuracy and sense-resistor power
dissipation. Most applications employ a current-
sense voltage of 50mV to 100mV. Choose a sense
resistor such that:
SENSE
______________________________________________________________________________________
I
(MIN)
PEAK
PEAK
V
SAG
=
V
) under conditions when V
SOAR
= Current-Limit Threshold Voltage / I
= I
K (V
is the peak inductor current.
=
LOAD(MAX)
OUT
2
= L
Determining the Current Limit
PEAK
×
C
✕
K (V
+ 0.075V) V
F
(
I
):
∆
×
PEAK
LOAD
I
DUTY V
OUT
+ [(LIR / 2)
LOAD MAX
2
minus half the ripple cur-
Transient Response
/ (2 C
+ 0.075V) V
(
(
OUT
IN MIN
(
OUT
)
)
IN
✕
2
+ min off - time
I
×
LOAD(MAX)
)
is small, V
V
L
-
OUT
V
IN
IN
OUT
- V
)
)
L(MIN)
OUT
OUT
]
dif-
is
7a). Use the worst-case value for R
MOSFET data sheet, and add a margin of 0.5%/°C for
the rise in R
ed R
the current-limit threshold voltage. If the default 50mV
threshold is unacceptable, set the threshold value as in
step 2 above.
In all cases, ensure an acceptable current limit consid-
ering current-sense and resistor accuracies.
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
Also, the capacitance value must be high enough to
absorb the inductor energy going from a full-load to no-
load condition without tripping the OVP circuit.
For CPU core voltage converters and other applications
where the output is subject to violent load transients,
the output capacitor’s size depends on how much ESR
is needed to prevent the output from dipping too low
under a load transient. Ignoring the sag due to finite
capacitance:
In non-CPU applications, the output capacitor’s size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
Figure 7. Current-Sense Configurations
DS(ON)
MAX1845
a)
and I
DS(ON)
CS
R
LX
DL
ESR
L(MIN)
R
ESR
with temperature. Use the calculat-
≤
Output Capacitor Selection
LIR
≤
from step 1 above to determine
I
×
LOAD MAX
V
I
V
P P
LOAD MAX
DIP
−
(
(
MAX1845
)
b)
)
DS(ON)
DL
CS
LX
from the